1. Field of the Invention
The present invention generally relates to a transmission module incorporated in a cellular telephone for mobile communications or the like.
2. Description of the Related Art
FIG. 1 shows the configuration of a conventional transmitter unit 40. The transmitter unit 40 amplifies the power of a radio frequency (high frequency) signal on which data are superimposed and outputs the amplified radio frequency signal through an antenna. The transmitter unit 40 has a SAW (Surface Acoustic Wave) filter 41, a driver amplifier 42, a power amplifier 43, a coupler 44, an isolator (circulator) 45, and a duplexer 46, which are connected in this order. The coupler 44 branches a part of the transmitted signal amplified by the power amplifier 43 into a detector 47 so that the transmitted signal is fed back to the power amplifier 43 for output stabilization.
A transmitted signal Tx is mixed with a local oscillation (LO) signal and is thus converted into the radio frequency signal. The driver amplifier 42 and the power amplifier 43 amplify the power of the radio frequency signal so as to have a given gain. The amplified radio frequency signal is output via an antenna 50.
The coupler 44 branches part of the transmitted signal amplified by the power amplifier 43 into the detector 47. Then, the detector 47 converts the transmitted signal (RF signal) from the coupler 44 into a voltage. The output voltage increases as the input power increases. The voltage output by the detector 47 is compared with a given signal level by a baseband unit 10, which controls the power amplifier 43 on the basis of the difference between the voltage output and the given signal level. The baseband unit 10 may be an IC device.
The duplexer 46 is a filter device that separates the transmitted signal to be sent via the antenna 50 and the frequency range of the received signal from each other. The duplexer 46 prevents the transmitted signal from traveling to the receiver side and prevents the received signal from traveling to the transmitter side. The isolator 45 outputs the transmitted signal from the coupler 44 to the duplexer 46, and prevents a component of the received signal leaked to the transmitter side from being applied to the power amplifier 43.
The isolator 45 functions to protect the power amplifier 43 from the leaked component of the signal received through the antenna 50. Generally, the isolator 45 employs a combination of ferrite magnets, and has a large size, which occupied a large circuitry area. In addition, power consumption takes place in the isolator 45.
Japanese Patent Application Publication Nos. 2000-295055 (Patent Document 1) and 8-307286 (Patent Document 2) show configurations of the transmitter circuits from which the isolators are eliminated. Patent Document 1 discloses a variable load circuit on a signal line between the output terminal of the power amplifier and the antenna. Further, there is provided a control circuit, which detects the direction and level of a variation in the load on the output side of the power amplifier and adjusts the load of the variable load circuit to compensate for the variation. Patent Document 2 discloses a directional coupler interposed between the power amplifier and the antenna. The output power of the power amplifier is controlled on the basis of the power of the reflected wave from the directional coupler.
The transmitter unit 40 shown in FIG. 4 is composed of the discrete circuit elements. This makes it difficult to achieve impedance matching in the transmitter unit 40 and needs troublesome work. Further, the isolator is essentially needed to cope with noise externally applied.